Move shadow node reference updates to tree commit (facebook#50753)

Summary:
Pull Request resolved: facebook#50753

Runtime Shadow Node Reference Updates (RSNRU) is currently implemented through the clone method which on each internal clone updates the runtime reference to point to the new clone. This guarantees that the runtime reference always points at the latest revision of the shadow node.

This came with the constraint that RSNRU could only run from one thread at all times, otherwise the React renderer state (current fiber tree) would end up being corrupted by receiving reference updates from multiple threads cloning shadow nodes.

This change moves the reference update step to the locked scope of the commit phase. Since the runtime is blocking on the commit and the scope is locked, it is safe and correct to update the runtime references with the latest revision of the shadow node after running state progression and layout.

By moving the reference update to the commit, we can support shadow node syncing from any thread since the actual runtime references are now executing at a safe time and the renderer state will stay valid at all times.

This change is gated behind the `updateRuntimeShadowNodeReferencesOnCommit` feature flag, which enabled shadow node syncing from any thread and reference updates only during the commit.

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D73038439

fbshipit-source-id: d90308498f3c0625dc87158f15311d1088aad8b0

Author: Nick Lefever

packages/react-native/ReactCommon/react/renderer/core/ShadowNode.cpp

@@ -322,13 +322,16 @@ void ShadowNode::transferRuntimeShadowNodeReference(
   destinationShadowNode->runtimeShadowNodeReference_ =
       runtimeShadowNodeReference_;
 
-  updateRuntimeShadowNodeReference(destinationShadowNode);
+  if (!ReactNativeFeatureFlags::updateRuntimeShadowNodeReferencesOnCommit()) {
+    updateRuntimeShadowNodeReference(destinationShadowNode);
+  }
 }
 
 void ShadowNode::transferRuntimeShadowNodeReference(
     const Shared& destinationShadowNode,
     const ShadowNodeFragment& fragment) const {
-  if (useRuntimeShadowNodeReferenceUpdateOnThread &&
+  if ((ReactNativeFeatureFlags::updateRuntimeShadowNodeReferencesOnCommit() ||
+       useRuntimeShadowNodeReferenceUpdateOnThread) &&
       fragment.runtimeShadowNodeReference) {
     transferRuntimeShadowNodeReference(destinationShadowNode);
   }

packages/react-native/ReactCommon/react/renderer/mounting/updateMountedFlag.cpp

@@ -7,6 +7,8 @@
 
 #include "updateMountedFlag.h"
 
+#include <react/featureflags/ReactNativeFeatureFlags.h>
+
 namespace facebook::react {
 void updateMountedFlag(
     const ShadowNode::ListOfShared& oldChildren,
@@ -47,6 +49,10 @@ void updateMountedFlag(
     newChild->setMounted(true);
     oldChild->setMounted(false);
 
+    if (ReactNativeFeatureFlags::updateRuntimeShadowNodeReferencesOnCommit()) {
+      newChild->updateRuntimeShadowNodeReference(newChild);
+    }
+
     updateMountedFlag(oldChild->getChildren(), newChild->getChildren());
   }